BACKGROUND: Adolescent onset bipolar disorder often presents with co-morbid disorders of which psychoactive substance use disorders are notable. Mania symptoms and co-morbid psychoactive substance use disorders prone adolescents with bipolar disorder to impulsivity, impaired judgment, and risk taking behavior which often includes sexual risk behavior. There are dearth of information on pattern of co-morbid disorders and sexual risk behavior in adolescent onset bipolar disorder in Nigeria. This study assessed the prevalence and pattern of co-morbid disorders and determined associated factors of sexual risk behavior among adolescents with bipolar disorder. METHODS: Socio-demographic information was obtained from the adolescents using socio-demographic questionnaire. Clinical interview, physical examination and laboratory investigations were employed to establish co-morbid disorders in these adolescents during the outpatient follow up visits over a one year period. RESULTS: A total of forty six (46) adolescents with bipolar disorder were followed up over a one year period. Twenty two (47.8%) of the adolescents had co-morbid disorders with cannabis use disorders, alcohol use disorders, conduct disorder with or without other psychoactive substance use accounting for 23.9%, 8.7%, 13.0% respectively and HIV infection, though a chance finding accounting for 2.2%. Twenty one (45.7%) of the adolescents had positive history of sexual risk behavior, which was significantly associated with presence of co-morbid disorders (p = 0.003), level of religion activities in the adolescents (p = 0.000), and marital status of the parents (p = 0.021). CONCLUSION: When planning interventions for children and adolescents with bipolar disorder, special attention may need to be focused on group of adolescents with co-morbid disorders and propensity towards impulsivity and sexual risk behavior. This may help in improving long term outcome in this group of adolescents.